Due to the growing number of people that live an inactive life, many physical activity promotion products and services have been developed over the last decades, both for research and commercial objectives. Said physical activity promotion products in most cases try to calculate or estimate the heart rate in order to display the person's pulse during a physical activity. At present, the most successful devices that measure the heart rate for athletes make use of a chest belt. These devices measure the electric signal of the heart (ECG) during the athlete's physical activity. However, these chest straps are uncomfortable to wear, which practically restricts their use to serious athletes.
Because more and more people are aware of the power of monitoring heart rate for their health, and most people try to avoid wearing such a kind of chest strap due to its uncomfortableness, the paradigm of measuring a heart rate slowly changes from high resolution and low comfort to medium resolution but higher wearing comfort.
This is achieved, for example, through optical heart rate monitors, which may be attached to different parts of the body, e.g. also to the wrist of the athlete. A device of this kind, which is known from the prior art, is commercially distributed under the name ePulse2™. This heart rate monitor includes an optical sensor that is similar to pulse oximeters available in the market. It is realized as an arm band that can be worn conveniently on the wrist.
Optical sensors used for heart rate measurement, however, suffer from large movement artifacts, especially in case of large and fast movements as these occur during a physical activity like running, cycling or rowing. That is because the optical sensor in fact optically measures the blood flow inside the blood vessel, which blood flow is, of course, also influenced by the body movement, so that discontinuous, rough movements occur within the blood vessel. This results in large movement artifacts, which complicate the heart rate measurement.
For this reason, some optical sensors known from the prior art use an additional motion sensor to measure the occurring motion of the body part and to compensate for the resulting motion artifacts. However, there is a limit. When the movement of the body part, to which the sensor is attached, becomes very large, the optical sensor does not provide reliable measurements anymore, even when the measurement signal is compensated, respectively adapted with the motion signal provided from the motion sensor.
In this case, the heart rate monitor either displays a wrong heart rate value or no heart rate value at all. This is regarded to be a major disadvantage, since the measurement results in an inaccurate or no displayed heart rate.
Several years of experience with a large set of users have shown that participants attach high importance to the reliability and the comfort of such heart rate measurements. In particular, when athletes or sportsmen have engaged in heavy activity or exercise they consider it necessary to have a reliable, real time feedback of the current heart rate at all times. If the heart rate monitor has registered a wrong heart rate or does not even display a value of the heart rate, this can be experienced as de-motivating and has a negative impact on the overall perception of the device.